Coating compositions, especially for metal substrates, that are applied prior to painting and then heat cured, are often used to impart corrosion resistance to metal surfaces as well as enhance adhesion of subsequently applied paints. Such compositions typically contain a metal in non-elemental form, which metal can exhibit multi-valency and is susceptible to valency reduction to a lower state during curing of applied composition. For example, in U.S. Pat. No. 3,671,331 there are disclosed chromium bonding compositions that contain a hexavalent-chromium-providing compound and reducing agent therefor in liquid medium.
Such compositions exhibit excellent adherence to metal substrates and offer many desirable characteristics, including the formation of a firm bond between subsequently applied topcoats and the underlying metal substrate. Some of these topcoats can be additional compositions containing a multi-valent metal in aqueous medium. In U.S. Pat. No. 3,718,509 there is disclosed a coating such as can be provided by U.S. Pat. No. 3,671,331, which coating is topcoated with a chromium bonding coating. Such topcoating can contain hexavalent-chromium-providing compound in a liquid medium containing water.
After the pre-paint coating and then the topcoat application and curing, a resulting coated article is most usually required to exhibit corrosion resistance for the coated substrate. In addition, prior to application of the topcoating, the initially coated metal should, most desirably, be water resistant. This water resistance not only maintains coating integrity during application of a subsequent coating, but also permits water quenching to be used in cooling the initially coated article. Water quenching can provide most economical cooling. This cooling is most desirable where the subsequent coating will be applied by immersion coating technique, and such subsequent coating composition is heat sensitive.